Since energy issues and global environmental issues are becoming more serious, solar cells are receiving more attention as an alternative energy source for replacing fossil fuels. In the solar cell, carriers (electrons and holes) generated by light irradiation of a photoelectric conversion section composed of a semiconductor junction or the like are extracted into an external circuit to generate electricity. A collecting electrode is provided on the photoelectric conversion section of the solar cell for efficiently extracting carriers generated in the photoelectric conversion section into the external circuit.
For example, in a crystalline silicon-based solar cell using a single-crystal silicon substrate or a polycrystalline silicon substrate, a collecting electrode made of a slender metal is provided on a light receiving surface. Also, in a heterojunction solar cell having amorphous silicon layers and transparent electrode layers on a crystalline silicon substrate, collecting electrode(s) are provided on the transparent electrode layer(s).
In a silicon-based thin-film solar cell using an amorphous silicon thin-film, a crystalline silicon thin-film or the like, and a thin-film solar cell such as a compound solar cell using CIGS, CIS or the like, an organic thin-film solar cell or a dye-sensitized solar cell, a transparent electrode layer is provided on a surface of a photoelectric conversion section on the light receiving surface side in order to reduce the surface resistance of the light receiving surface. In this configuration, the transparent electrode layer can perform a function as a collecting electrode, and therefore in principle, it is not necessary to specially provide a collecting electrode. However, since conductive oxide, such as indium tin oxide (ITO) or zinc oxide, forming the transparent electrode layer has a resistivity higher than that of metal, there is a problem where the internal resistance of the solar cell increases. Thus, a collecting electrode (metal electrode as an auxiliary electrode) is provided on the surface of the transparent electrode layer to improve current extraction efficiency.
The collecting electrode of the solar cell is generally formed by pattern-printing a silver paste by a screen printing method. This method is simple in terms of the process itself, but has a problem where the material cost of silver is high, and the resistivity of the collecting electrode increases because a silver paste material containing a resin is used. For decreasing the resistivity of the collecting electrode formed of a silver paste, it is necessary to thickly print the silver paste. However, since the line width of the electrode increases with the increase of the print thickness, thinning of the electrode is difficult, and the shading loss by the collecting electrode increases.
For solving these problems, a method is known in which a collecting electrode is formed by a plating method excellent in terms of material and process costs. For example, Patent Documents 1 to 3 disclose a solar cell in which a metallic layer made of copper or the like is formed by a plating method on a transparent electrode layer that forms a photoelectric conversion section. In this method, first, a resist material layer (insulating layer) having an opening section matching the shape of a collecting electrode is formed on the transparent electrode layer of the photoelectric conversion section, and a metallic layer is formed at the resist opening section of the transparent electrode layer by electroplating. Thereafter, the resist is removed to form a collecting electrode having a predetermined shape.
Patent Document 3 discloses that the line width of a plating electrode is made equal to or less than that of an under-layer electrode by forming the plating electrode layer using a mask after a formation of the under-layer electrode. In addition, Patent Document 3 discloses that a plating solution deposited on a substrate is washed off by water after a plating, an organic solvent or the like, in view of the problem that solar cell characteristics are degraded if the solar cell, on which a plating solution remains, is exposed under a high-temperature and high-humidity environment.
A method has been also proposed in which a self-assembled monolayer is used in place of a conventional resist material when a metallic pattern is formed by a plating method. The self-assembled monolayer (abbreviated as SAM) refers to a monolayer formed such that when a so-called self-assembling compound is adsorbed (chemisorbed) on a specific base material surface, the self-assembling compound is almost regularly arranged on the base material surface due to its molecular assembling property (adsorption action by an adsorption functional group and intermolecular interaction by a group bound to the adsorption functional group).
For example, Non-Patent Document 1 discloses a method in which a self-assembled monolayer is patterned by micro-contact printing, and the patterned self-assembled monolayer is used as a very thin resist to form a metallic pattern. Non-Patent Document 1 describes an electroplating method as one of the methods for forming a metallic pattern.
Patent Document 4 discloses a method in which a self-assembled monolayer is formed on a base material, the self-assembled monolayer is then partially peeled off, and an area where the self-assembled monolayer is peeled off is subjected to electroplating using the remaining self-assembled monolayer as an insulator (protective layer), so that partial plating is performed. As a method using a self-assembled monolayer for the patterning of a metal or the like for the formation of a transistor, Patent Document 5 discloses a method in which an etchant is spread over an unassembled region of a self-assembled monolayer to remove only a desired region of a base layer situated immediately below the unassembled region. In this method, a patterned layer having a property of inhibiting absorption of a self-assembled monolayer is formed on a predetermined region on a base layer, and then a self-assembled monolayer is formed on the base layer. This method takes advantage of the fact that an unassembled region of the self-assembled monolayer is obtained on the base layer in the vicinity of the patterned layer.
A technique using a self-assembled monolayer in relation to the formation of a collecting electrode of a solar cell, Patent Document 6 discloses a method in which a self-assembled monolayer is formed on a transparent electrode layer, and a collecting electrode is formed thereon to enhance the adhesion of the collecting electrode. In Patent Document 6, the collecting electrode is formed by a printing method, and the self-assembled monolayer is provided merely for enhancing adhesion with the collecting electrode. That is, Patent Document 6 does not relate to a technique for providing a collecting electrode by a plating method, and does not disclose patterning of a self-assembled monolayer, or the like.